Scanning electron microscope

Abstract

In a scanning electron microscope, slimming is reduced by reducing a frame count. As the frame count is reduced, the amount of detected secondary electrons decreases, so that a probe current amount is increased to emit an increased amount of detected secondary electrons. A primary electron beam is scanned on a sample, a histogram is created, and the histogram is second-order differentiated to calculate a level of halftone at which a sample image changes in contrast, and to calculate the probe current amount. By adjusting the frame count suitable for the calculated probe current amount, and the contrast suitable for the sample image, the slimming of the sample is limited, and a highly visible sample image is generated for length measurement.

Description

BACKGROUND OF THE INVENTION[0001]The present invention relates to a scanning electron microscope for observing a miniature pattern to measure dimensions thereof, and more particularly, to a scanning electron microscope for observing and measuring a sample, the shape of which can be deformed by an electron beam irradiated thereto.[0002]Scanning electron microscopes (SEM) are widely used in manufacturing and testing steps of a functional product such as a semiconductor device, a thin film magnetic head, and the like, which are fabricated by micro-machining the surface thereof, for measuring widths of processed patterns and inspecting the appearance of resulting products. The scanning electron microscope is an apparatus for forming the image of a sample by narrowing down an electron beam emitted from an electron source with a converging lens or an objective lens which makes use of an interaction of a magnetic field or an electric field with the electron beam, one-dimensionally or two-d...

AI Technical Summary

Benefits of technology

[0007]It is an object of the present invention to provide a scanning electron microscope which is capable of reducing the influence of slimming to produce a highly visible sample image when it is used to observe or measure a sample, such as an ArF resist, which is vulnerable to electron beam irradiation and can suffer from slimming.

[0011]However, when the frame count is reduced for reducing the slimming, an insufficient number of times of scanning for creating a sample image causes a smaller amount of detected secondary electrons, which would give rise to such problems as lower contrast and the generation of noise due to a lower S / N ratio. To avoid these problems, the present invention increases the probe current amount, which does not significantly affect the slimming, to detect an increased amount of secondary electrons emitted from a sample, thereby ensuring an image quality equivalent to a sample image generated by the conventional method, even though the frame count is reduced (hereinafter called the “low frame scanning scheme”).

[0012]In the prior art, a histogram is created for a generated sample image, the average of the histogram is defined as the brightness of the sample image, and the brightness is controlled by increasing the dynamic range of an amplifier. Then, the standard deviation of the histogram is defined as the contrast of the sample image, and the contrast is controlled by a detecting condition of a detector. With this conventional control method, when the low frame scanning scheme is performed, the histogram largely shifts due to large variations in the probe current amount, so that the sample image must be scanned repeatedly to control the histogram, and the histogram must be created again. Even if the slimming is reduced by reducing the frame count, the repeated scanning of the electron beam, for creating the histogram, results in an increased amount of slimming. A need therefore exists for a method of reducing surplus electron beam scanning by minimizing the number of times the histogram is created and calculating a brightness and contrast optimal for a sample image.

[0018]The electron beam microscope of the present invention creates a histogram of a sample image, and can find a minimum frame count from a calculated probe current amount based on halftone levels near the border of a sample and a background. By observing the sample under the conditions of the calculated probe current amount and the minimum frame count, a highly visible image can be displayed while limiting the slimming of the sample. In addition, the throughput can be improved by the low frame scanning scheme with the minimum frame count and the histogram control method of the present invention.

Problems solved by technology

However, the results of recent investigations have revealed that the ArF resist is highly vulnerable to electron beam irradiation, and when a formed pattern is observed or measured with a scanning electron microscope, the scanning of a converged electron beam causes a condensation reaction in a base acrylic resin or the like, resulting in a reduction in volume (hereinafter called “slimming”) and an eventual change in the shape of a circuit pattern.

However, a reduction in the irradiation density of an electron beam causes a reduction in the amount of secondary electrons generated from the sample, resulting in a dark image.

Disadvantageously, the prior art method of reducing the slimming does not take into consideration an image control technique which relies on the relationship between the number of times of electron beam scanning required for creating a sample image (hereinafter called the “frame count”) and the probe current amount, and fails to sufficiently reduce the slimming and generate a highly visible sample image.

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